1. Field of the Invention
The present invention relates to a lighting device and a lighting system that allows a plurality of light sources to light up at different luminance levels from each other.
2. Description of Related Art
A plurality of button switches for operation setup have been provided on electric equipments such as a car-mounted audio device or air-conditioner. In order to recognize the button switches even in the dark, a lighting system having a plurality of light-emitting diodes (LEDs) functioning as light sources in a lighting device has been widely used. Also in a small-sized liquid crystal display device used for, especially, a mobile phone, a lighting system provided with a plurality of LEDs acting as a backlight is in widespread use.
As a circuit configuration of a lighting device that allows the plurality of LEDs to light up, the circuit configuration as shown in, for example, FIG. 1, in which light-emitting diodes LED 1 and LED 2 are connected in series to each other, is used in general. More specifically, since the luminance levels of the LED 1 and LED 2 are set depending on a value of flowing current, a resistor R11 for restricting current is connected in series to the serially-connected LEDs 1 and 2 for luminance setup. This circuit configuration allows the luminance levels of each of LEDs to vary in accordance with a change in a voltage value without using a constant-voltage circuit, simplifying the circuit configuration.
A curved surface is often used to provide comfort in, for example, automobile interior. Button switches of car-mounted electric equipments are accordingly provided along the curved surface in some cases. On the other hand, a lighting system is constituted by mounting the LEDs 1 and 2, as well as electric parts on a flat substrate. As a result, when the lighting device is used as light sources of the button switches disposed along the curved surface, distances from each of the button switches to the light sources become different from each other, with the result that illuminance levels of the button switches differ form each other. Therefore, the luminance levels of the light sources must be set at different levels in order for illuminance levels of the button switches to be equal to each other.
In order to set the luminance levels of the LEDs 1 and 2 in the lighting system shown in FIG. 1 at different levels, it can be considered that a serial circuit of the LED 1 and a resistor R12 for setting luminance level and another serial circuit of the LED 2 and a resistor R13 for setting luminance level are connected in parallel as shown in, for example, FIG. 2 to make values of current flowing in the LED 1 and LED 2 different from each other. However, in the circuit configuration shown in FIG. 2, a plurality of serial circuits in which the resistors R12 and R13 for setting the luminance levels of the LEDs 1 and 2 are serially connected to the LEDs 1 and 2, respectively are connected in parallel, which increases the size and complexity of the circuit configuration more than that shown in FIG. 1.
In order to cope with this problem, it can be considered that a circuit configuration as shown in FIG. 3 in which the LEDs 1 and 2 are serially connected is employed to individually set the luminance levels of the LEDs 1 and 2. That is, as shown in FIG. 3, the resistors R12 and R13 are connected to the serially-connected LEDs 1 and 2, respectively in parallel. Resistance values of the resistors R12 and R13 are set to shunt current appropriately. Forward currents of the LEDs 1 and 2 are then set for differentiating luminance levels thereof. However, as is the case with the circuit configuration shown in FIG. 2, decrease in a voltage value to be applied makes the timings for lights (LEDs) to go off different since a constant-voltage circuit is not used. That is, when a large resistance value is set in the resistor R12 (R13) for higher luminance, current easily flows through the LED 1 (LED 2) connected in parallel to the resistor R12 (R13), so that reduction in luminance is not very noticeable. On the other hand, when a small resistance value is set in the resistor R13 (R12) for lower luminance, a current does not easily flow through the LED 2 (LED 1) connected in parallel to the resistor R13 (R12) to increase reduction in luminance. Therefore, when a voltage change occurs, variation in illumination may be observed due to difference in the timings for the LEDs 1 and 2 to go off. Therefore, a constant-current circuit must be provided, which may complicate the device configuration.
As described above, the conventional lighting system that allows a plurality of light-emitting diodes LED 1 and LED 2 to light up has a problem that the size and complexity of the circuit configuration that allows a plurality of light-emitting diodes LED 1 and LED 2 to light up at different luminance levels and prevents the timings for the LEDs 1 and 2 to go off from being varied may be increased.